1. Field of the Invention
The present invention relates to a conductive anti-reflection film that functions as an anti-reflection film and protects an AEF (Alternating Electric Field) from taking place, a fabrication method thereof, and a cathode ray tube having the conductive anti-reflection film formed on an outer surface of a face panel of a face plate.
2. Description of the Related Art
In recent years, it is pointed out that an electromagnetic wave generated in the vicinity of an electron gun and a deflection yoke of a Cathode ray tube used in TV sets and computers leaks out and may adversely affect an electronic unit disposed therearound.
To prevent the cathode ray tube from leaking out the electromagnetic wave (electric field), it is necessary to decrease the surface resistance of the face panel thereof. In other words, Japanese Patent Laid-Open Application Nos. 61-118932, 61-118946, and 63-160140 disclose various surface treatment methods for preventing a face panel from being statically charged. With such methods, the alternating electric field (AEF) can be prevented from leaking out.
To prevent the face panel from being statically charged, the sufficient surface resistance of the conductive film is around 1.times.10.sup.11 ohms/.quadrature. or less. However, with such a surface resistance, the AEF cannot be prevented from taking place. To prevent the AEF from taking place, the surface resistance of the conductive film should be 5.times.10.sup.2 ohms/.quadrature. or less.
Examples of the method for forming a conductive film with a low surface resistance are gas phase methods such as PVD method, CVD method, and sputtering method. For example, Japanese Patent Laid-Open Application No. 1-242769 discloses a method for forming a low resistance conductive film corresponding to the sputtering method. Since the gas phase method requires a large scaled machine for forming a conductive film, the investment cost for the machine is high. In addition, this method is not suitable for quantitative fabrication.
Moreover, the lower the specific resistance of a conductive material composing a conductive film, the higher conductivity can be obtained. Thus, when a conductive film containing metal particles is used, the AEF can be effectively prevented from taking place.
However, generally, even if a film containing metal particles is thin, it absorbs visible light. Thus, when the film is thick, the transmissivity of light in a short wave length region (blue region) decreases. Consequently, the luminance of the cathode ray tube decreases. When a conductive film is composed of only metal particles without a binder, since the bond force of the metal particles is insufficient, the film hardness becomes low. In contrast, when a conductive film is composed of metal particles with a binder, the resistance of the conductor film becomes high. Thus, sufficient conductivity cannot be obtained.
As another related art reference, Japanese Patent Laid-Open Application No. 6-208003 discloses a two-layered conductive anti-reflection film having a first layer that is a high refractive conductive layer containing conductive particles with a refractive index of 2 or more and a second layer that is a low refractive silica layer with a refractive index of 2 or less, the second layer being disposed on the first layer. In the two-layered conductive anti-reflection film, a light absorbing substance such as a coloring matter is contained so as to cause the color of the reflected light to be neutral and thereby suppress the reflected light from being colored. However, since the refractive index and reflectivity of the conductive layer containing metal particles are high, only with the light absorbing characteristics of the light absorbing substance, it is difficult to suppress the reflected light from being colored.
A method for forming a transparent conductive film is known as coating method or wet method. In this method, a solution in which transparent and conductive particles are dispersed is coated on a substrate and thereby a coat film is formed. The coat film is dried and hardened or sintered. For example, a solution of which particles of tin oxide containing Sb (ATO) or particles of tin oxide containing In (ITO) and a binder of silica (SiO.sub.2) are mixed and dispersed is coated on a substrate and thereby a coat film is formed. The coat film is dried and hardened or sintered and thereby a transparent conductive film is obtained. In such a transparent conductive film, conductive particles (of ATO or ITO) mutually contact and thereby conductivity is obtained. It is known that the conductive particles mutually contact by the following mechanism.
Just after the coat film has been formed on the substrate, the conductive particles do not mutually contact. Silica as a binder is present in a gel state between each conductive particle. By sintering the coat film at a temperature of 200.degree. C., the silica in the gel state is closely and densely formed. In this process, individual conductive particles mutually contact each other. Thus, the conductivity of the conductive particles is obtained.
Although the transparent conductive film formed in such a manner is conductive, since much insulation binder component of densely formed silica is present between each conductive particle, sufficient conductivity that prevents the AEF from taking place cannot be obtained.
To solve such a problem, Japanese Patent Laid-Open Application No. 8-102227 discloses a method for forming a transparent conductive film that prevents the AEF from taking place. The transparent conductive film is formed in the following manner. A solution in which conductive particles that do not contain polymer binder component are dispersed coated on a substrate. Thus, a first coat film containing the conductive particles is formed. Thereafter, a second coat film containing a silica binder or the like is formed on the first coat film. Thereafter, the first and second coat films are sintered at the same time. Thus, a transparent conductive film that has conductivity necessary for preventing the AEF from taking place is formed. In this method, when the silica gel contained in the second coat film is sintered and thereby densely formed, the first coat film is also densely formed. Thus, the conductive particles mutually contact each other and thereby sufficient conductivity can be obtained. When the solution containing the silica binder or the like is coated on the substrate, the binder slightly penetrates into the first coat film. However, since the amount of silica that penetrates into the conductive particles is small in comparison with the case that a mixture of conductive particles and silica binder is coated on the substrate, it is expected that the conductivity is improved.
However, in such a method, when the first and second coat films are sintered, since the second coat film is more contracted than the first coat film, the conductive particles contained in the first coat film are unequally densified. Consequently, since a portion of which conductive particles do not mutually contact each other takes place, as the conductive film, sufficient conductivity cannot be obtained.